There is known a so-called Ziegler-Natta catalyst as a catalyst for polymerization of an olefin such as ethylene, propopylene and butene-1, which catalyst is obtained by contacting a solid catalyst component obtained using a transition metal compound of the Groups 4 to 6 of the Periodic Table with an organoaluminum compound.
However, a process for producing a particulate ethylene-propylene block copolymer using such a Ziegler-Natta catalyst, which process comprises the steps of (1) polymerizing propylene alone in a gas phase to obtain a crystalline polypropylene, and (2) copolymerizing ethylene and propylene in a gas phase in the presence of the obtained crystalline polypropylene to obtain an amorphous ethylene-propylene copolymer rubber, has a problem that the block copolymer particles per se adher more easily when a content of the ethylene-propylene copolymer rubber in the block copolymer increases, and as a result, it is difficult to produce the block copolymer. Although the above-mentioned copolymer is practically a mixture of the propylene homopolymer and the ethylene-propylene copolymer, said copolymer is called a block copolymer in the field of polyolefins.
For example, JP-A 1-115909 discloses a process to solve the above-mentioned problem, which process comprises the step of using a porous substance at the time of production of the solid catalyst component for olefin polymerization. However, the solid catalyst component obtained by said process has a problem that polymerization activity per said component is markedly low, and as a result, productivity of the olefin polymer is low.